It is known to have inspection or viewing windows or ports in opaque walls, panels or other structures to allow a person on one side to see through to, or to inspect conditions on, the other side. For example, in the context of protective metal housings enclosing electrical equipment, such as electrical power transformers, a housing may include one or more small windows so that the condition of the electrical equipment inside the housing can be inspected visually, by infrared thermography (the technique of producing images from the normally invisible infrared spectrum) or by ultraviolet imaging techniques from the exterior of the housing. The ability to make such inspections through a window, without having to remove the housing itself, is important for safety reasons, so that inspectors can make routine inspections of the equipment in operation yet without danger of being exposed to open live electrical equipment, such as electrical shock, arc flash or mechanical movement.
The inspection windows are generally mounted in fixed positions in one or more panels (e.g. sides, top, front, back) of the housing, typically with the plane of the window in or close to the plane of the panel in which the window is mounted. Such inspection windows are located so that the particular portion of the equipment inside that is of interest or concern can be inspected. If it should transpire that a problem arises in a section of the equipment that not visible through an inspection window, then it is unlikely that routine inspections through the inspection windows would reveal same.
The inspection windows can include any suitable transparent material with suitable physical properties (e.g. thickness, size, toughness, heat resistance) that would protect an inspector from potentially harmful conditions inside the housing. Typical examples for visual inspection would be glass and transparent plastics (e.g. polycarbonate). Sometimes, if only basic mechanical protection is required, the window opening could be protected only by an open grille. Examples for infrared thermographic inspection would be various mineral crystals, such as calcium fluoride, germanium, zinc selenide or sapphire (depending on precise application).
One of the potential conditions in electrical equipment that routine inspections seek to identify is overheating, which could be indicative of a more serious underlying problem. For example, electrical connections can become loose with time and, if not identified and repaired in timely manner, could lead to overheating, electrical arcing, short circuiting and a potentially dangerous equipment failure. As compared to connections in good condition, such loose connections tend to cause overheating of the material in the vicinity of the loose connection and as a result that material emits greater amounts of infrared radiation. Overheating can be caused by various other conditions, including broken or oxidized wiring, improper grounding, unbalanced loads and system overloads.
Although infrared radiation cannot be detected by the human eye, infrared cameras (such as those sold by FLIR Systems under the trade-mark FLIR-570 or by Fluke under the trade-mark FLU-T120) and similar equipment are capable of sensing, measuring and recording infrared radiation. Through the use of such instruments, the presence of overheating, or ‘hot spots’, in the electrical equipment can be identified and appropriate remedial action taken thereafter.
Unfortunately, conventional glass and plastic window materials tend to be opaque to infrared and ultraviolet radiation in varying degrees. Accordingly, such materials cannot be used in an inspection window if it is intended to measure such radiation from the other side of the window. In recognition of these difficulties, special materials supported by special window structures (all designed to allow the passage of such radiation) have been developed and put into use. For example, for infrared applications, such windows are sold by Square D, a division of Schneider Electric, under the trade-mark H VIR COMET and by Global Maintenance Technologies or GM Tech of Chelmsford, UK under the trademarks VP-50 SERIES, VP-75 SERIES and VP-100 SERIES. These latter windows include a filter element made of either calcium fluoride, germanium, zinc selenide or sapphire (depending on precise application), all of which are sufficiently transparent to infrared radiation but in some cases may be opaque to visible light. As shown in the Prior Art FIG. 1, the filter is held by an annular frame across which the filter (and, for some filter materials, a supporting protective latticework) are positioned. In addition, the manufacturer of this device includes, and recommends usage of, a protective cover, to be opened only during an inspection operation. The use of such a window allows an infrared camera or detector to look into the housing to detect and measure the infrared radiation emitted by the equipment within view.
Typically, the physical properties of such special filter materials limit the field of view that any particular window can have. First, the mechanical properties of the filter material (even though supported by a latticework structure) and cost factors may limit the physical size of the window to the order of several inches. Second, in the case of infrared filter materials, the optimum viewing angle through such filters is typically about plus or minus 30 degrees from the vertical to the plane of the filter, although some useful information can be extracted for about an additional 15 degrees on either side. However, if the viewing angle exceeds about 45 degrees from the vertical, the transmission of infrared radiation through the filter falls off significantly. In addition, as the viewing angle increases from 45 degrees, the geometry of the filter and its supporting structure may interfere with a clear line of sight. In short, for various reasons, the field of view of such windows can be relatively limited.
If there are large-size or multiple zones of equipment that ideally should be monitored by inspection, because of the small physical size and the limited field of view of the viewing windows described above, multiple windows may have to be used. As each window is relatively expensive, the overall cost of manufacturing and operating housings can be substantially increased. Alternatively, to keep costs down, fewer windows may be used but with a consequent reduction in ability to inspect effectively and with potential safety implications.
There thus remains a need for a viewing window structure which will allow convenient and safe inspection of a larger zone of the interior of a housing.